The OCaml typedtree (post-typing AST) is rather intricate and contains
details about the type inference process that you might not want to have to
know about.
If you don't need the full width of the OCaml language features, I would
recommend starting from a smaller codebase. Rewriting a parser for a subset
of the OCaml language is easy, type inference difficulty depends on the
features you have in your language, but there are numerous smaller projects
you could reuse.
See for example [MinCaml], a minimalistic compiler for teaching purpose, or
the [poly] language from the Programming Language Zoo.
[MinCaml] http://min-caml.sourceforge.net/index-e.html
[poly] http://andrej.com/plzoo/html/poly.html